This came up again as a question in support, so I wanted to give the question an answer on the forum.
The easiest thing to do is ctrl-z back to when you had the footprints … you can always undo all the way back to beginning of the design.
if you have moved forward with your design and just one or two are missing… you can press the foot in the parts list in the right hand bar (screenshot below) the footprint will re-appear in the layout.
Cheers,
Michael